Concentrating by removing the solvent
Some separations cannot be done with a filter or a centrifuge because the material is dissolved. Salt in water, sugar in juice and a chemical product in a solvent are all examples. To recover or concentrate the dissolved material, a plant can remove some of the liquid. An evaporator does this by heating the solution so part of the solvent turns to vapor and leaves, which makes the remaining solution more concentrated. A crystallizer goes one step further and arranges conditions so the dissolved material forms solid crystals that can then be separated.
The two are often found together in a line, with the evaporator doing most of the bulk concentration and the crystallizer finishing the job.
How evaporators work
At its simplest, an evaporator is a vessel with a heat source. The solution is heated, vapor is released into a separation space, and thick liquid is drawn off. The heat usually comes from steam passing through tubes or a jacket. Since boiling water takes a lot of energy, much of the engineering in evaporators is about using that heat sensibly.
Common styles include:
- Falling-film evaporators, where the liquid flows in a thin film down the inside of heated tubes. They suit heat-sensitive liquids because contact time is short.
- Rising-film evaporators, where liquid boils as it rises in tubes.
- Forced-circulation evaporators, which use a pump to push liquid through the heater at speed. These can handle liquids that tend to form deposits or crystals.
- Plate evaporators, which use plate-type heating surfaces and are compact.
- Agitated thin-film evaporators, which use a rotating wiper to spread viscous material over a heated wall.
The choice depends on how thick the liquid becomes, whether it is sensitive to heat, whether it foams, and how likely it is to leave deposits.
Saving energy
Because evaporation is energy hungry, plants often reuse vapor. In a multiple-effect arrangement, vapor from one stage heats the next stage, which runs at lower pressure and temperature, so a single input of steam does the work of several stages. Another approach is vapor recompression, where vapor is compressed to raise its temperature so it can be reused as the heating medium. Both ideas aim to reduce steam use, and they connect to broader topics like waste heat recovery and the supply of process steam. Exact savings depend on the design and the duty.
Running under vacuum lowers the boiling temperature. That helps protect heat-sensitive products and lets low-grade heat do useful work, but it adds vacuum equipment. Vacuum pump types vary, and the choice is made with the rest of the system.
How crystallizers work
Crystals form when a solution holds more dissolved material than it normally can at its current conditions, a state called supersaturation. A crystallizer creates that condition, usually in one of three ways: cooling the solution so solubility drops, evaporating solvent so concentration rises, or sometimes adding another substance that reduces solubility. Tiny crystal nuclei appear first, and then they grow as more material deposits on them.
What operators care about is crystal size and quality. Rapid supersaturation tends to create many small crystals, while slower, controlled conditions give fewer, larger ones. Large, uniform crystals are usually easier to separate, wash and dry. Typical designs include cooling crystallizers, evaporative crystallizers with a circulation loop, and draft-tube designs that keep crystals suspended gently. Mixing intensity and residence time also influence results, so industrial agitators and circulation pumps are part of the design.
After crystallizing, the slurry usually goes to a centrifuge or filter, then to dryers. Mother liquor left behind may be recycled or sent for treatment.
Typical problems
- Scaling and fouling. Dissolved minerals and organics can deposit on heating surfaces, lowering heat transfer. Heat exchanger fouling explains the general mechanisms, and evaporators often need scheduled cleaning.
- Foaming and carryover. Droplets entrained in the vapor can contaminate condensate, so separators and mist eliminators are used.
- Corrosion. Concentrated salts and heat can attack materials, so material selection matters.
- Crystal control. Unwanted fine crystals, caking, or crystals sticking to walls can disrupt operation.
- Controls. Level, temperature, pressure and density measurements keep the process stable, which is where process control sensors come in.
Safety
These units involve steam, hot liquids, pressure or vacuum, and sometimes hazardous chemicals. Follow the manufacturer’s manual, use lockout and tagout style procedures in general terms, and ensure trained, certified personnel handle servicing and entry into vessels. Vessels may be pressure equipment with legal inspection requirements, depending on location. This article is general information, not engineering advice.